Static switching arrangements of the cross-point type



2 Sheets-Sheet 1 W. B. DELLER ETAL Matth 30, 1965 Filed Aug. 28, 1961 March 30, 1965 w, B. DELLER ETAL 3,176,273

sTATlc swITcHING ARRANGEMENTS oF THE cRoss-PoINT TYPE Filed Aug. 28, 1961 2 Sheets-Sheet 2 F'i Z United States Patent O This invention relates to static switching arrangements of the cross-point type such as may be employed in switching systems, for example automatic telephone exchange systems, affording a plurality of possible communication paths which can be taken into use, as required, to establish connections between terminal circuits.

In general a static switching arrangement of the crosspoint type as herein considered basically comprises two groups of conductors which constitute, in eiiect, a twoco-ordinate array of conductors forming a plurality of so-called cross-points corresponding to the different combinations of two conductors obtained by taking the conductors one from each group at a time. At each of these cross-points, a so-called cross-point circuit, which comprises one or more circuit components of a static nature, includes connections to the two array conductors that form the particular cross-point and serves as a static switching element having an off (or high impedance) condition and an on (or low impedance) condition so far as the transmission or" communication currents (for example speech currents) between these two conductors is concerned. A complete switching system may comprise a number of tandem connected switch stages each comprising a number of these basic co-ordinate or cross-point switching arrangements.

A static switching component which is suitable for use in a'static switching arrangement of the cross-point type is a form of semi-conductor device known as a four-Zone (or four-layer) diode. Such a device is a two-terminal device which has a delinite offj or high impedance, condition and a delinite on, or low impedance, condition. Change from the high to the low impedance condition can be efected by the application ot a breakdown (or ring) Voltage to the device which thereafter can be held in the low impedance condition by maintaining a relatively low holding current through it, but which reverts to the high impedance condition immediately upon the cessation of such holding current.

Where four-zone diodes are employed as the static switching components in a two-co-Ordinate array, crosspoint selection may be etiected by applying respectively tothe two array conductors that form the cross-point, substantially equal amplitude positive-,going and negativegoing tiring pulses, the amplitude of each of these pulses being at least one-half of the amplitude of the breakdown voltage required to change the four-zone diode at the cross-point to its low impedance condition. With this method of selection all the cross-point diodes connected to the array conductor receiving the positive-going tiring pulse andv likewise all the cross-point diodes connected to the array conductor receiving the negative-going firing pulse will have one-half of the breakdown voltage appearing across them, but only the particular diode at the crosspoint concerned has the full breakdown voltage across it and therefore only this diode is changed to the on condition to establish a low impedance connection between the two array conductors. Upon the cessation of the tiring pulses a maintaining circuit established in respect of the selected cross-point provides a holding current for maintaining the four-zone diode in the on condition.

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With presently known processes of manufacture, four- Zone diodes having a relatively low breakdown voltage are more readily manufactured than those having a high breakdown voltage. Furthermore, the four-zone diodes produced have breakdown voltages which vary over a relatively wide range with respect to a nominal value. Thus if a static switching arrangement of the cross-point type could be designed so that four-zone diodes having breakdown voltages which vary, as between the diodes themselves, over a relatively wide range about a fairly low nominal value would be acceptable for use thereon, then a greater proportion of such diodes as manufactured could be utilised with consequent substantial economic advantage. However, as will be considered in some detail hereinafter, when static switching arrangements using four-zone diodes are employed in some practical forms of switching systems, standing voltages of appreciable amplitude may be present on their array conductors as a result of the holding current owing in the maintaining circuits established in respect of selected cross-points, and these standing voltages can necessitate the use of cross-point diodes having a higher breakdown voltage than would otherwise be required in order to minimise the possibility of spurious cross-point selection caused by an applied firing pulse being effective in conjunction with a standing voltage to produce a voltage of breakdown amplitude across a diode.

With a View to enabling cross-point diodes having a lower breakdown voltage to be used for a given maximum standing voltage, cross-point selection in a static switching arrangement of the cross-point type using four-zone diodes is achieved in accordance with the present invention by applying to the two array conductors concerned respective positive-going and negative-going firing pulses which are unequal in amplitude but which together give a combined voltage at least as great as the breakdown voltage of the diodes, the respective amplitudes of the pulses and their points of application to the arrangement being such that either of these pulses, in the absence of the other, can produce across any cross-point diode, in conjunction with any standing voltage likely to be present in the arrangement, a voltage which is significantly less than the breakdown voltage for the diode.

In order that the invention may be more fully understood reference will now be made to the accompanying drawing in which:

FIG. 1 is a circuit showing several tandem-connected static switching arrangements employing four-zone diodes, through which a communication path can be established; and

FIG. 2 is a diagram illustrating the voltages present at various points in a communication path established in the circuit of FIG. l.

Referring to FIG. 1, two terminal circuits LC1' and LC2 suitable for transmitting and receiving A C. intelligence (speech) currents are interconnected over a communiaction path (shown in heavy line) which is established through four tandem-connected static switching arrangements S1 to S4. For the sake of simplicity there is shown in the switching arrangement S1 only tWo conductors X and XX of one of the two groups of array conductors concerned, and only two conductors Y and YY of the other of these two groups, together with the four cross-points A, B, C and D pertaining to these conductors; but it will of course be appreciated that there may in practice be ten or more conductors in each of the two groups and correspondingly one hundred or more cross-points. The switching arrangement S4 is similarly represented by two pairs of conductors X', XX and Y', YY and the appertaining cross-points P, VQ, R and S, while the intermediate switching arrangements SZand S3 are represented only by ther particular A'cross-points- (M and N) byV which the communication path isex-y f tended through them. Each of Ythe cross-points referred to is constituted by an lindividual four-Zone diode., V

The terminal circuit LC1 is associated with the array conductor Y in the switching arrangement S1 andcomprises a coupling transformer T1 having two windings (I, II) ,for coupling a pair offspeech wires splto the conductor Y, anisolating `rectilier Rf1 for preventing voltages of positive potentialon therconductor Y from s feeding back into the terminal circuit LCI, a choke coil L1 for preventing A KC. intelligence currents Afeeding into a maintaining Yvoltage supply through its positive terminal e -i-Vm, and a decouplingr capacitor C1' for decoupling thev Y A.C. intelligence currents to earth. 1 Connected :across the winding` (II) of the couplingtransf0rmer-T1 is al double Zener diode Dfi which serves to limit the Ypeak f' voltage produced by A.C..intelligence currents to its ownY break-down voltage. The' linef circuit LCZ similarly comprises a coupling transformer 1T2 for couplingv a pair of speech wires, sp2v to the conductor YY' and having a doubleZen'er diode DfZ connected vacross itslwindingY (Il), and an isolating rectier RfZ. There isfno chokeA coil or decoupling capacitor in the circuit LC 2`which, however, includes a signalling resistor Rsl across which being Vspuriously operated to its low `irrnpcdance, condition. For example, consider the establishment of another communication -path *betweenV two furtherterminal circuits LC3 and LC4 using the cross-points C and S (together with two cross-pointsnotj shown in the* switching arrangements S2 and S3). If, in the switching arrangement S1, in accordance with existing practice, a negativegoinfg firing pulse ,-vprof amplitude -1/2Vs is applied to the array conductor XX and a'positive-going -iiring pulse -l-vpkof.. amplitude -l-l/zVs is Vapplied to thearray conductor YY, to operate the diode at the cross-point-C .to itslowr impedance condition, thenthe algebraic sum of .the voltages -Vs and v(Vx-l-vs) across the diode at thecross-point B could fspuriouslyjoperate this diode to its'low impedancepcondition, However, in applying vthe present invention thene'gative-going iringfpul'se -vp is made lessv in amplitude thanl/zVs by a certain voltage, say Vz, vwhilethe' positive-going firing pulse -|`vp is made greater injamplitude than l/zVs by this voltage Vz. In consequence, whereas thevoltage across the diode at thefcross-point C is still Vs,y thevo'ltage across the diode at cross-point-B will'be the algebraicfsum of voltages (-tl/aVsL-l-Vz) and (Vx-Hfs) ,which is avoltage smaller r by the voltage Vz than the voltage which was previously signalling "voltages can bedeveloped for utilisation inv any suitable manner.

The established communicationV path includes the cross-point diodes A, M, N and P, andiV three isolating rectiers Rf, Rf4 and RJSr located respectively between v adjacent switching arrangements.

With the communication path established as shown by the application "of appropriate tiring pulses; -l-vp and` Y -vp to thepa'rticular array conductors such as X,Y and form), a holding current for maintaining the crossfpoint X', YY' (pulseapplying circuitsfor. this purposerbeing Y indicated by the rectangles PC andtaking any suitable 35 diodes A, M, N andA P in theirrlow impedance condition 1 flows through ,the pathfrom the terminal I-l-Vmfin the circuit-LCl Vto earth in the circuit LCZ. Due tolthisflow of holdingcurrent a potential gradient is set up between one end ofthe path andthefother,resulting inistanding Y voltages being present at various points along the path.

The points of,standing'potentialwhich are of importance l wtih regard'to'what vminimum value Vof breakdown volte age fourzone diodes suitable foruse as the cross-'point diodes can have, are the' point x at -the high potential end of the path'and thepoint y atthelow potentialend. ',As

will be seen` in the diagram shown in FIG. 2, the rpoint x is at a standing potential Vxwhich is equal to Vm minus' s the voltage v1 dropped across the componentsof the circuit LC1, and the point y is at a standing potentialfVy whichis equal to Vx minusthe-voltage v2. dropped' across l j breakdown voltagetrof 80' volts areV required and that,

' since Vp-Y-VSO-l-l-JSIL,volts,'positivegoing andrne'gativethe series-connected cross-point diodes A, M, N and Pf and the interveningisolating rectifle'rs Rf3, Rf4, andY RJS.

Equivalently, the standing potential "Vy is'equ'al to the voltage v3dropped acrossrthe components ofthe circuit 1 LCZ. In order that the maintaining voltagefcandrive -a 1 current 'ofs holding magnitude through the components connected in, or in serieslwith, the `communication path,

' the voltage -Vm may have to'be fairly high: this`means' that' the voltages Vx Vand Vy are proportionatelyv high.

Moreover, Athe flow of AA.C. intelligence currentsin the,y

communication path can vary ythe :voltages Vx and Vy byv a value l-i-vs, wherevvskis the peak voltage produced by these currents; y v Y p L The total possible voltage VJc-I-vsthat can exist at the .and due tothe standingfpotential at the point y:

Y Vxvequals 9 volts, Vy equals 'y1 volt, and vs equalsi volts. Also thereis a tolerance ofsay, 15% in the breakdownvolt'age Vs of thefeross-point diodes to be taken intoraccount, so Ithat a ringfvoltage Vp of suitable breakdown magnitude'for the diodes has to be not Itrhas been found that for'reliable `operation'of a static switching` arrangement the maximum voltage whchshould .'be allowed across a cross-point diodewhichhas 'not to break Ydowiu ,should not; exceed 757% of the minimum tolerance value of Vs (i;e; 75% Vs min). Y Y,

Thus, in the case of'ring pulses of equal amplitude as previously used:V s Y v equating (1) and. volts. j l Y t This means that `four-zone diodes4 having a nominal going ringpulses (gl-yp), cach for satisfactory operation. l Y Y Where tiring pulsesrofunequal amplitude are usedrin of 42 volts are required accordance with the present invention, let the vclnltage lof by Vz volts. Y, Y v

point" x may be(V ofthe order of one-half of the breaky down ,voltage .(l/zVs) rof a certain type of vcross-pointy diode thatfmight be used in the static switching arrange-V ments -and therefore` since this voltage' is present at one side of `all Ythe -free Vcross-pointKV diodes, such `as the one f at B,which arel connected to the; conductor Y, a negative-n going firing-pulse. of one-half ofV ,theA breakdownvoltage applied to the other array conductor to whichany onew of these'diodes is connected couldfresult in'this YdiodeY .the vnegative-going firing pulses be reduced by Vz volts and thevoltage of the positive-going pulses be increased Thus, in v this v instance, thepoint ,xz l

' i/zV Vp-l-Vn-(t/Hvmsu Vs min. (s)

due to, they standing rvoltage at l Vsl Y i 3/ (V 20 f With the present invention therefore, four-zone diodes having a nominal breakdown voltage of only 54 volts, as compared with 80 volts in the former example, may be employed. It is evident that the tiring voltage Vp will be about 57 volts, and with Vz=5 volts, the positiveand negative-going tirng pulses -l-vp and -vp will be about 23 volts and 33 volts respectively. From the foregoing it will be seen that, in general, the amount of asymmetry between the amplitudes of the tiring pulses is about half the difference between the amplitudes of the maximum and minimum standing voltages liable to be encountered.

For the sake of simplicity the above numerical examples do not take into account other tolerances which may in practice have to be allowed.

What we claim is:

1. In a circuit arrangement comprising a plurality of coordinate switching networks each of which comprises a rst array of conductors, a second array of conductors disposed in coordinate relationship with said rst array and defining cross-points therewith, and a plurality of cross-point switches constituted by respective twoterminal four-zone diodes each connected between the pair of conductors defining its cross-point; means associated with each switching network for rendering conductive the diodes at selected cross-points therein thereby to establish connection between the conductors delining such cross-points but also resulting in the establishment of standing voltages on these conductors, said means including first pulse means for selectively applying a irst switching voltage pulse of one polarity and amplitude to a selected conductor in one of said first and second arrays and second pulse means for selectively applying a second switching voltage pulse of opposite polarity and different amplitude relative to said irst pulse to a selected conductor in the other of said arrays, the amplitudes of said rst and second pulses being together sul'licient to produce across the diode connected between said selected conductors a combined voltage which is at least as great as the diodes forward breakdown switching voltage, but the amplitude of each such pulse being insufficient in relation to said standing voltages to cause breakdown of another diode connected between the conductor to which such pulse is applied and another conductor on which such standing voltage may be present.

2. A circuit arrangement as defined in claim l wherein one of said rst and second pulse means produces a switching voltage pulse which differs in amplitude from the switching voltage pulse produced by the other of said pulse means by an amount approximating to half the difference between the maximum and minimum amplitudes of the standing voltages liable to be present.

References Cited by the Examiner UNITED STATES PATENTS 2,945,855 7/60 Hussey 179-18.7 2,951,124 8/60 Hussey et al. l79-18.7 2,951,125 8/60 Andrews 179-18.7 3,011,155 1l/6l Dunlap 340-166 X 3,020,353 2/62 Heetman 179-18] 3,027,427 3/62 Woodin 179-18.7 3,033,936 5/62 Simms 179-18.7 3,047,667 7/62 Hussey 179-18] ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner. 

1. IN A CIRCUIT ARRANGEMENT COMPRISING A PLURALITY OF COORDINATE SWITCHING NETWORKS EACH OF WHICH COMPRISES A FIRST ARRAY OF CONDUCTORS, A SECOND ARRAY OF CONDUCTORS DISPOSED IN COORDINATE RELATIONSHIP WITH SAID FIRST ARRAY AND DEFINING CROSS-POINTS THEREWITH, AND A PLURALITY OF CROSS-POINT SWITCHES CONSTITUTED BY RESPECTIVE TWOTERMINAL FOUR-ZONE DIODES EACH CONNECTED BETWEEN THE PAIR OF CONDUCTORS DEFINING ITS CROSS-POINT; MEANS ASSOCIATED WITH EACH SWITCHING NETWORK FOR RENDERING CONDUCTIVE THE DIODES AT SELECTED CROSS-POINTS THEREIN THEREBY TO ESTABLISH CONNECTION BETWEEN THE CONDUCTORS DEFINING SUCH CROSS-POINTS BUT ALSO RESULTING IN THE ESTABLISHMENT OF STANDING VOLATAGES ON THESE CONDUCTORS, SAID MEANS INCLUDING FIRST PULSE MEANS FOR SELECTIVELY APPLYING A FIRST SWITCHING VOLTAGE PULSE OF ONE POLARITY AND AMPLITUDE TO A SELECTED CONDUCTOR IN ONE OF SAID FIRST AND SECOND ARRAYS AND SECOND PULSE MEANS FOR SELECTIVELY APPLYING A SECOND SWITCHING VOLTAGE PULSE OF OPPOSITE POLARITY AND DIFFERENT AMPLITUDE RELATIVE TO SAID FIRST PULSE TO A SELECTED CONDUCTOR IN THE OTHER OF SAID ARRAYS, THE AMPLITUDES OF SAID FIRST AND SECOND PULSES BEING TOGETHER SUFFICIENT TO PRODUCE ACROSS THE DIODE CONNECTED BETWEEN SAID SELECTED CONDUCTORS A COMBINED VOLTAGE WHICH IS AT LEAST AS GREAT AS THE DIODE''S FORWARD BREAKDOWN SWITCHING VOLTAGE, BUT THE AMPLITUDE OF EACH SUCH PULSE BEING INSUFFICIENT IN RELATION TO SAID STANDING VOLTAGES TO CAUSE BREAKDOWN OF ANOTHER DIODE CONNECTED BETWEEN THE CONDUCTOR TO WHICH SUCH PULSE IS APPLIED AND ANOTHER CONDUCTOR ON WHICH SUCH STANDING VOLTAGE MAY BE PRESENT. 